1. Field of the Invention
The present invention relates to compositions for the internal sizing of paper and, more particularly, it relates to ketene dimer/nonreactive hydrophobe compounds providing improved sizing results.
2. Description of the Prior Art
The main component of paper and paperboard is cellulose fiber. The flat web of cellulose fibers may contain inorganic fillers, starch, pigments and other papermaking adjuvants. Such paper and paperboard would readily absorb aqueous liquids. This property would be a serious disadvantage when the paper is used in printing or coating or pasting operations. Also, most papermaking machines apply a surface coating to the semidried paper using an aqueous coating mix at a size press. The application of a surface coating to a paper or board as above is technically difficult, especially at the lighter weights of paper.
These technical difficulties have been overcome by sizing the paper and paperboard. Sizing agents are used to impart to the paper and paperboard resistance to aqueous penetrants. Various types of sizing agents have been used commercially over many years. Most end use applications for the paper require that the paper is sized internally--that is the sizing agent is added to the paper components before the paper web is formed.
Ketene dimer sizing agents were introduced to the paper industry in the late 1950's and early 1960's. These allowed for the first time the production of internally sized paper and paperboard under neutral to alkaline pH conditions. Traditionally clay had been used as the filler but now chalk could be used within the neutral/alkaline papermaking conditions. Paper and paperboard made under these conditions has many commercial advantages, and the use of ketene dimer sizing agents has now spread throughout the worldwide papermaking industry. Ketene dimers are water insoluble products and they are used largely in the form of aqueous dispersions which are added to the papermaking stock.
After the wet web of paper has been formed on the papermaking machine it is dried by passing around a series of heated cylinders. This period of heating and drying promotes a chemical reaction between the ketene dimer and the hydroxyl groups on the cellulose fiber, possibly also with hydroxyl groups on the fillers. This chemical reaction is time and temperature dependent. On some papermaking machines the duration of heating is sufficient to promote the chemical reaction to such an extent that a sizing effect results on the machine. This, however, is not the case on most papermaking machines since they are operated at maximum speed to optimize paper production and this reduces the period of the heating and drying. Consequently most papermaking machines using ketene dimer sizing agents alone do not make sized paper on-machine. This detracts from the operation of the size press. The chemical reaction between dimer and hydroxyl groups does continue in the dried paper but it may take several days to reach naturally its full sizing development. This slow development of sizing creates problems with the conduct of further operations such as printing, coating, pasting, etc.
This problem of the slow development of sizing with ketene dimers alone has received considerable attention over the years. A solution used commercially since the early 1970's has been to employ a promoter resin with the ketene dimer. Promoter resins of dicyandiamide/formaldehyde condensates have been used successfully to speed up the development of sizing. Another potential solution is to employ with the ketene dimer another sizing agent which will give an immediate effect on-machine. One such additional sizing agent is wax as proposed in Japanese Patent J58 087395. Others such as pentaerythritol aliphatic acid esters, polyalkylene glycol di-aliphatic acid esters, mono-and/or di-fatty acid esters of alkane diols, polyvalent metal salts of fatty acids, fatty cane sugar esters and polyalkylene glycol mono-fatty acid esters have been proposed in Japanese Patents J58 091895, J58 091894, J58 087396, J57 112499, J57 101096 and J57 101095 respectively.
Japanese Patent J57 112498 proposes the use of mixtures of ketene dimers with di- and/or triglycerides as being sizing agents that can be used in neutral and alkaline conditions and which give a sizing effect in a short time. The appropriate amounts to use are 5-100 parts of glyceride, preferably 10-50 parts of glyceride, relative to 100 parts of ketene dimer to give degrees of sizing in a short time of approximately 50-68 percent of the degree of natural cure after one day. The use of these mixed size systems does not increase the level of sizing after one day above that achieved by the use of ketene dimer alone.
A further disadvantage of a ketene dimer sizing agent is that it can react with water to yield and ineffective ketone. This action reduces the efficiency of the sizing agent.
The object of the present invention is to provide a sizing agent that includes the use of a ketene dimer within its composition that sizes paper and paperboard within a short time and improves the efficiency of the ketene dimer.